Pix would help. It sounds like you have an old horn with the dried core still in it, inside looks like a thick Spider Web. Once dried it is a little more difficult to get the core out but it can be done. Boiling is probably the best way, do it out side even if its dry. Let it boil for an hour or so and test it to see if it will move, long Needle Nose pliers help, each one is different. It may take awhile but it will come out. Once it is out you can measure the depth of the cavity, cut and drill. Some horns are just not Powder Horn material.
Any horn, raw of polished needs to be worked down. A course rasp is a good starter, followed by a finer file or two. Once you get it to the shape you want, a scraper with finish it off. Sand paper can be used if you like, after the files start with 80 and work your way to about 220, 80, 120, 180, 220. Even then a scraper can be used to get the finish you want.
I am not a fan of Highly Polish horns, once again to each his own.
